KINGDOM – PROTISTA
Living organisms included in Protista are as follows:
Term protista by C. Cuvier and Kingdom established by E.Hackel :
All the organisms included in Protista are unicellular (acellular) eukaryotes.
Mode of nutrition in protista is of different types. viz,
(1) Holophytic or Photosynthetic:- They synthesize their own food through photosynthesis.
Fig: Autotrophs: Synthesize their own food
(2) Holozoic: Some protists have holozoic mode of nutrition, which is similar to animals i.e. food is first ingested and then digested.
(3) Absorptive: Some protists obtain their food from dead organic substances. These protists secrete some extracellular enzymes.
These enzymes convert the complex organic substances into simpler substances. Now, these simple substances can be easily absorbed through the body surface.
(4) Mixotrophic: Some Protists have both holophytic and saprophytic type of nutrition.
Protists reproduce Asexually and Sexually
1. Asexual Reproduction: This is the most common method of reproduction in protists. Asexual reproduction takes place in favourable conditions.
It is of following types-
(a) Binary fission: Two daughter cells are formed by the division of one mother cell. After this each daughter cell grows to form a normal organism.
(b) Spore formation: Some protists have special structure known as sporangia. Spores are formed in this sporangia. The sporangia bursts after sometime and all the spores become free. These spores form a new cell after germination.
Fig: Spore formation in Sporangia
2. Sexual Reproduction: Sexual reproduction was first of all seen in protists. In sexual reproduction two haploid gametes fuse to form a diploid zygote. This process is known as syngamy.
Syngamy is of three types:
(a) Isogamy: It is the easiest way of sexual reproduction. In isogamy the fusing gametes are morphologically (i.e. shape, size, structure) similar but physiologically(i.e. functionally or genetically) they may be similar or dissimilar. when fusing gametes are physiologically dissimilar, process is called physiological anisogamy.
Fig: Fertilization or syngamy
(b) Anisogamy: The fusing gametes are morphologically dissimilar (smaller - larger, motile - immotile) but physiologically they may be similar or dissimilar.
(c) Oogamy: It is the developed form of anisogamy. Male gamete is small and motile while female gamete is large and non-motile. This female gamete is known as egg. The formation of male & female gametes takes place in sex organs.
Life Cycle of Protist
(1) Life cycle showing zygotic meiosis:–
Note: In this type of life cycle the zygotic phase is only diploid and remaining all the phases are haploid so this type of life cycle is known as haplontic life cycle.
(2) Life cycle showing gametic meiosis: When Protist is diploid and meiosis takes place during gamete formation, then it is called gametic meiosis.
In this type of life cycle during sexual reproduction, meiosis takes place in diploid cell , due to which haploid gametes are formed. Now haploid gametes fuse to form diploid zygote. And after that mitosis takes place in zygote, due to which diploid cells are formed again.
Eg. Noctiluca, Diatoms, Acellular slime moulds.
Note : In this type of life cycle only gametic phase is haploid and remaining all phases are diploid so this type of life cycle is known as diplontic life cycle.